# How to make BJT/MOSFET work in RF?

I am trying to design an amplifier with a transistor model. I followed some basic tutorials online (http://microwave.eecs.utk.edu/ECE545_files/02_Lab_2.pdf) and designed a transistor model for BJT/MOSFET. I am stuck in making the amplifier work in RF frequencies. I guess either I am using a low-frequency model or improper circuitry and not sure which one of

Now, for MOSFET/BJT I fixed a bias point (for maximum transconductance in MOSFET/ fix Beta,VBE,IC,IB in BJT) and then designed the biasing circuitry and simulated the S-Parameter/AC simulation. I am getting the S21/Gain dropping to 0dB near 150 MHz. I guess this has to do with transit frequency.

I guess two mistakes - either I am using a low-frequency model or improper circuitry and not sure which.

My circuit looks like the one below for BJT:

From the datasheet of the 2N3904 I got that for maximal RF performance (current gain bandwidth product = 300MHz) one must have Ic=10mAdc and Vce=20Vdc. This is not the case in your circuit.

• I tried channging the Ic and Vce, it does not work, 0dB around 200MHz. I am expecting something in GHz, preferably in double digits. Sep 9, 2018 at 19:19
• This transistor has a max. current gain bandwidth of 300MHz, this is a no-go for frequencies above 300 MHz. Sep 9, 2018 at 19:22
• Also, I was using 2N696 model, whose datasheet says about RF performance, but could not find any gain curves or s parameter data in RF range. Sep 9, 2018 at 19:24
• I got it, so the transistor model is wrong. Thanks!, which answers my question. Sep 9, 2018 at 19:24

From some vague memory, I recall the gain of a transistor is

Z(Cfeedback + Cout) * gm.

Thus when gm is 0.01 (or 1/100 ohms, or 0.26 milliamp for a bipolar), and the Z(collector total capacitance) is 100 ohms, you'll have a Voltage gain of 1.

This is ignoring "base transit time" and other aspects.

At lower frequencies, you should have more gain, set by Early Voltage and Zin of the next stage, and by Zout of the prior stage, and the collector/drain biasing resistors.